To prevent the erased files from being recovered, click Security Options, use the slider to choose how many times to write over the erased data, then click OK.

Writing over the data three times meets the U.S. Department of Energy standard for securely erasing magnetic media. Writing over the data seven times meets the U.S. Department of Defense 5220-22-M standard.

Click Erase, then click Done.

Note: With a solid-state drive (SSD), secure erase options are not available in Disk Utility. For more security, consider turning on FileVault encryption when you start using your SSD drive.